1. Field of the Invention
This invention relates to a welding process and a photovoltaic device. More particularly, it relates to a process for welding i) a semiconductor-holding member having a thin-film semiconductor layer on its surface and ii) a metal member, in particular, welding a conductive substrate of a photovoltaic device and a conductive foil member. It also relates to a photovoltaic device produced by such a welding process.
2. Related Background Art
Photovoltaic devices on which light is made incident to cause an electromotive force are utilized in various scenes. Especially in recent years, as we are getting more concerned with environmental problems, our higher hope is being placed on their application to clean-energy solar cells.
At present, solar cells are roughly grouped into a crystal type making use of single-crystal silicon or polycrystalline silicon, an amorphous type and a compound semiconductor type. Amorphous silicon solar cells can not be equal to crystal type solar cells just on conversion efficiency, but can be made large-area with ease and also have such a great absorptivity coefficient that they can be operated in thin film, having advantageous features the crystal type solar cells do not have. Thus, they are one of solar cells considered promising.
What presently keeps solar cells from spreading is that the solar cells require a high production cost. In order to lower the production cost of solar cells, it is presented to;
(1) efficiently utilize electricity-generating regions; PA1 (2) lessen connecting points to achieve the reduction of connecting-area material cost and the reduction of personnel expenditure for such connection; and PA1 (3) lessen expenses for forming photovoltaic layers. From these viewpoints, it is necessary and indispensable to make solar cells of a large area.
FIGS. 1A and 1B are diagrammatic views showing an example of solar cells; FIG. 1A is a diagrammatic plan view as viewed on the light-receiving side, and FIG. 1B a diagrammatic cross-sectional view along the line 1B--1B in FIG. 1A.
A photovoltaic device 100 shown in FIGS. 1A and 1B is made up by superposing a lower electrode layer 103, a semiconductor layer 104 and a transparent electrode layer 105 in order on a substrate 102 of stainless steel or the like. As the transparent electrode layer 105, a transparent conductive film of indium oxide or the like is formed which also serves as a reflection preventive means and a current collection means.
The transparent electrode layer 105 is partly removed in lines as shown by reference numeral 101 (etching lines) in FIG. 1A, using an etching paste containing FeCl.sub.3, AlCl.sub.3 or the like which is coated by a method such as screen printing followed by heating. The transparent electrode layer 105 is partly removed so that any short which may occur between the substrate 102 and the transparent electrode layer 105 when the periphery of the photovoltaic device is cut away may not adversely affect the effective light-receiving regions of the photovoltaic device.
Collector electrodes 107 are also formed on the surface of the photovoltaic device 100 so that the electric power generated can be collected with good efficiency. In order that the electric power generated at the semiconductor layer 103 can be delivered without loss, the collector electrodes 107 are formed by bonding metal wires (e.g., copper wires coated with carbon paste) onto the transparent electrode layer 105; the wires having been thinly coated with a conductive adhesives. Such copper wires are used in order to lessen current loss by using a material having a high conductivity.
A conductive foil member 108 is further provided as an additional collector electrode for these collector electrodes 107. Beneath the conductive foil member 108, an insulating member 109 is provided in order to ensure the insulation from the etching lines 101 whose performance is not assured.
In the photovoltaic device fabricated in this way, the conductive foil member 108 and substrate 102 function as terminals of both poles, through which the electric power can be delivered.
However, this device alone can not be usually used for the generation of electricity. Usually, a single electricity-generation cell generates too low of voltage, and hence it is necessary to connect devices in series to achieve a high voltage.
FIGS. 2A and 2B illustrate photovoltaic devices connected in series which are constituted of the device shown in FIG. 1A (an instance of two devices in series) A conductive foil member 110 of one photovoltaic device and a substrate 111 of another photovoltaic device adjoining thereto are connected by means of, e.g., a connecting member 112 made of copper foil, to connect the photovoltaic devices electrically in series. To connect these, for example a solder containing a flux for stainless steel may be used to carry out soldering, followed by cleaning with a solvent such as MEK (methyl ethyl ketone) to complete series connection.
When, however, it is attempted to make the above conventional solar cells large-area, not only the area is merely made larger but also the following problems may occur in respect of conversion efficiency.
(1) Since the quantity of generated electric currents becomes larger and the collector electrodes 107 become longer, the resistance loss (i.sup.2 R) increase to cause a decrease in conversion efficiency.
(2) Since the path length of electric currents increases when a conductive substrate having not so good conductivity such as a stainless steel substrate is used, the resistance loss (i.sup.2 R) increases to cause a decrease in conversion efficiency.
As a means for solving these problems, for example, U.S. Pat. No. 5,667,596 discloses a method that can deliver the electric power without any decrease in conversion efficiency even in large-area solar cells by providing at part of the conductive substrate a conductive foil member by ultrasonic welding or soldering.
In the ultrasonic welding, however, the following problems (1) to (3) should be taken into consideration, and in the soldering the following problems (4) and (5).
(1) When metals of an iron type and a copper type are welded, a difference in their lattice constants makes elements of both mix insufficiently, so that their joint strength may lower especially in the case of the ultrasonic welding that joins only the surface layers. What can be noted as its example is an instance where a copper foil member which is inexpensive and has good conductivity is welded to a photovoltaic device having a substrate of stainless steel.
(2) Since the metals are joined only at the surface layers, a low joint strength may result when the metal surfaces stand oxidized. In addition, such joint strength may further become lower in an environment of high temperature and high humidity (85.degree. C., 85% RH) to enable no assurance of long-term reliability.
(3) Since the welding conditions must be changed depending on how the surfaces of both metals stand oxidized, the ultrasonic welding can not be said to be much suited for mass production.
(4) When heat is applied with a soldering bit, the heat may cause the substrate to deform to come to have no flatness of the photovoltaic device.
(5) The flux may be removed insufficiently even with use of a solvent, so that the device may rust in a condition having temperature and humidity. As the result, cover materials of the photovoltaic device may come off.